Suitable sheet metal aluminum alloys may be formed at elevated temperatures by hot blow forming, hot stamping, or the like into intricate three-dimensional shapes. Often the formed articles are inner and/or outer closure panels for automotive vehicles. In each of these elevated temperature processes, a preheated aluminum alloy sheet is formed between opposing forming dies carried on platens of a hydraulic press. The forming surfaces of the forming dies are typically machined from cast blocks of a suitable tool steel alloy. And the forming surfaces of a die are finished (e.g. polished) to a very smooth finish, especially where the surface of the part must present an attractive finish to a user.
In hot blow forming, a highly formable aluminum alloy sheet (e.g., AA5083) is heated (at e.g., about 500° C.) and gripped at peripheral edges between complementary opposing dies. Pressurized air or other fluid is applied against one side of the sheet to stretch it into conformance with the forming surface of one die. The opposing die provides an air chamber on the pressurized side of the aluminum sheet. Both dies may be heated to elevated forming temperatures to maintain the sheet at a predetermined forming temperature for shaping of the sheet. The sheet may first be pressed against one die for pre-shaping, and then blown against the opposing die for finish shaping. Thus, at least one surface of the hot sheet is stretched against and over the forming surface of a die.
In production operations, heated sheet workpieces are repeatedly placed on the press, formed on the heated die(s), and removed. A lubricant, such as boron nitride, is applied to the sheets to buffer the repeated sliding, frictional contact. But, particularly in regions such as die radii where local pressures will be high, the aluminum will locally weld to the die creating small patches of aluminum on the die. These small patches, once formed, promote additional die-aluminum interaction and will, as more aluminum sheets are processed, grow to relatively large, layered particles of, primarily, aluminum but also incorporating aluminum oxide and boron nitride which adhere to the die surface. Within any given high pressure region, the particles may be formed and may vary in size; but a typical aluminum alloy particle may, for example, have dimensions in the 100 to 200 micron range. This die-adhering debris causes indentations, scratches and other defects in formed parts. In many hot formed parts the surfaces will be visible to users and surface defects like this cannot be tolerated.
The hot stamping of aluminum alloy sheet materials typically uses different aluminum alloys. Somewhat lower forming temperatures than in hot blow forming may be employed (for example, 300° C. to about 400° C.). And the heated dies are configured so that one die pushes the heated metal against an opposing die. But again dry lubrication material and aluminum fragments from the sheet metal workpieces combine to form die-adherent debris on the dies which must be removed.
U.S. Pat. No. 6,516,645, titled Hot Die Cleaning for Superplastic and Quick Plastic Forming,” describes the use of solid carbon dioxide pellets for removal of dried lubricant, such as boron nitride, from hot die surfaces. Sheet forming production is interrupted and an air stream carrying the pellets is systematically directed over and against the forming surfaces of the die or dies. The impact of air, carbon dioxide, and CO2 pellets (from which carbon dioxide gas is subliming) impinging on the surfaces sweeps away lubricant material in a clean and efficient manner that does not damage the hot die surfaces. But this cleaning method does not effectively remove aluminum metal particles or debris from the die surfaces. It has been necessary to remove the dies from service, allow them to cool, and to scrape the aluminum debris from the surfaces, using manual polishing and grinding. Often the forming surfaces required further polishing before they could be returned to production use.
A practice is needed for removal of aluminum metal particles from hot die surfaces without prolonged interruptions of production of formed parts.